Blood borne myeloid cells (macrophages and dendritic cells) comprise a major component of the inflammatory infiltrates in patients with multiple sclerosis (MS) and animals with experimental autoimmune encephalomyelitis (EAE). They have been implicated in the demyelination and axonal loss that cause disability in these disorders. However, relatively little is known about how myeloid cells are regulated in the periphery and CNS during autoimmune demyelinating disease. We have found that myeloid progenitor cells are mobilized from the bone marrow at an accelerated rate immediately prior to EAE exacerbations. Newly exported Ly6ChiCD11b+ monocytes infiltrate the CNS during the preclinical stage and give rise to the CD11c+MHC ClassIIhi DC that constitute a significant percent of neuroinflammatory cells during the symptomatic stage. In Aim 1 we will investigate the cytokine pathways and molecular mechanisms underlying the expansion and mobilization of bone marrow myeloid cells during EAE. Our working hypothesis is that GM-CSF and MIP-1a, induced by myelin-specific T cells, stimulate bone marrow stromal cells to produce G-CSF and CXCL1/2. These factors, in turn, activate resident neutrophils to secrete proteases that degrade chemokines and adhesion molecules critical for the sequestration of hematopoietic precursor cells in intramedullary niches. Interruption of any step in this pathway (ex, by neutralizing G-CSF or by inactivating proteases) will prevent myeloid cell release and ultimately exhaust the peripheral monocyte pools that provide a source of CNS infiltrating cells during relapses. In Aim 2 we will investigate the factors that stimulate Ly6ChiCD11b+ monocytes to differentiate into CD11c+MHC Class IIhi DC within the CNS. We propose that CNS infiltrating monocytes acquire characteristics of myeloid dendritic cells consequent to direct interactions with myelin- specific T cells. The roles of candidate soluble factors (such as GM-CSF and TNF) and cell surface molecules (such as RANKL, CD40L and Lymphotoxin-) will be assessed. Our studies are likely to provide insights into the pathways driving the mobilization and development of pathogenic myeloid cells during autoimmunity. The results are expected to suggest novel therapeutic targets and biomarkers in MS related to myeloid cell dysregulation.